1. Technical Field
The present invention relates to a system for detecting optical signals reflected off a fiber Bragg grating; and more particularly, to a detection system for sensing peaks in the wave formation of optical signals reflected off a fiber Bragg grating.
2. Description of Related Art
Fiber Bragg gratings (FBGs) have been utilized in a variety of applications, including the use of FBGs as sensors. When configured as a sensor, a wide variety of optical configurations can be used to read the spectral information from the FBG.
FIG. 1 shows a fiber Bragg grating sensor system generally indicated as 10, having a broadband source 12, a coupler 14, a first FBG 16, a second FBG 18 and an optical detection unit 20. The broadband source 12 provides a broadband optical light that covers the resonance wavelengths of the FBGs 16, 18.
In operation, the broadband source 12 illuminates the FBGs of interest. The FBGs provide reflected optical signals containing reflected spectral information that is then directed by the coupler 12 into the optical detection unit 20, which performs an optical-to-electrical signal conversion and is responsible for determining each of the wavelengths reflected by the FBGs 16, 18.
FIG. 1C shows a typical return spectrum from the two FBGs 16, 18. Several techniques for optical detection have been developed including methods which effectively read the optical spectrum range of interest and present this information for processing. Also present on the return spectrum is an overall background signal which can be present in the returned spectra when reflections are present in the optical path. Once this spectra is detected, the FBG peaks can be identified and the wavelength of each FBG reflected signal calculated. However, as shown, a background signal will be present in the returned spectra often due to optical connectors, splices or imperfections in the FBG itself. Also, small Fabry-Perot cavities can create modulations on the reflected signals which can appear to be very similar to the reflections from the FBG sensors of interest. These background and unwanted signals can often make it very difficult for a system to determine the location of a valid FBG signal from the unwanted background signals. Because of this, a fixed threshold cannot be accurately used across the entire wavelength band due to potential back-reflected spectral signals, as shown in FIG 1C.
The present invention provides a method and apparatus for detecting a valid FBG signal on a returned optical spectra.
The apparatus includes a fiber Bragg grating peak detection system having a fiber Bragg grating parameter sensing and detecting system in combination with a variable threshold or grating profile peak detection unit.
The fiber Bragg grating parameter sensing and detecting system responds to a physical parameter, such as pressure or temperature depending on the application, for providing a fiber Bragg grating parameter sensing and detecting signal containing information about the physical parameter being sensed.
The variable threshold or grating profile peak detection unit responds to the fiber Bragg grating parameter sensing and detecting signal, for providing a variable threshold or grating profile peak detection unit signal containing information about a peak detected in the fiber Bragg grating parameter sensing and detecting signal that is used to determine the physical parameter.
In particular, the fiber Bragg grating parameter sensing and detecting system includes a broadband source, a coupler, one or more fiber Bragg gratings, and an optical detection unit. The broadband source provides a broadband optical signal via the coupler to the one or more fiber Bragg gratings. The one or more fiber Bragg gratings respond to the broadband optical signal, and further responds to a physical parameter, including temperature or pressure, for providing a fiber Bragg grating optical signal containing information about the parameter via the coupler to the optical detection unit. The information communicated in the fiber Bragg grating optical signal is in the form of one or more peaks associated with one or more wavelengths. The optical detection unit responds to the fiber Bragg grating optical signal, for providing an optical detection unit signal.
The variable threshold or grating profile peak detection unit detects one or more peaks using either a variable threshold peak detection or a grating profile peak detection, or a combination thereof. During the variable threshold peak detection, the variable threshold or grating profile peak detection unit determines a respective local threshold value for each wavelength over a spectral band of the fiber Bragg grating optical signal. During the grating profile peak detection, the variable threshold or grating profile peak detection unit determines whether there is a grating profile by looking at the points on each side of the local peak.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing, which are not drawn to scale.